Add carrier overview and checkout documentation (#295)

* Add checkout and routing rules documentation

* Update link

* Add first carrier overview

* Remove incomplete carriers and unsupported services

* Update checkout and routing rules concept

* Add budbee to carriers

* Update routing rules example

_includes/navs/concepts_nav.html

@@ -50,5 +50,13 @@
         <li class="nav--level-2"><a href="#automatic-deactivation">Automatic deactivation</a></li>
       </ul>
     </li>
+    <li class="mt-m ml-0">
+      <a href="#checkout">Checkout / Routing Rules</a>
+      <ul class="nav">
+        <li class="nav--level-2"><a href="#checkout-introduction">Introduction</a></li>
+        <li class="nav--level-2"><a href="#checkout-routing-rules">Routing rules</a></li>
+        <li class="nav--level-2"><a href="#checkout-additional-ressources">Additional Ressources</a></li>
+      </ul>
+    </li>
   </ul>
 </nav>

_includes/navs/reference_nav.html

@@ -87,5 +87,11 @@
         <li class="nav--level-2"><a href="#deleting-a-webhook">Delete</a></li>
       </ul>
     </li>
+    <li class="mt-m ml-0">
+      <a href="#routing-rules">Routing Rules</a>
+      <ul class="nav">
+        <li class="nav--level-2"><a href="#get-shipping-options">Get shipping options for a route</a></li>
+      </ul>
+    </li>
   </ul>
 </nav>

assets/images/logos/carriers/60x/dpd_nl.png

@@ -0,0 +1 @@
+dpd.png

assets/images/logos/carriers/60x/dpd_nl@2x.png

@@ -0,0 +1 @@
+dpd@2x.png